In recent years, the following automobile navigation systems (hereafter, referred to as in-vehicle terminals) have been mounted in vehicles: automobile navigation systems provided with statistical traffic information generated based on traffic information including situations of the past traffic jams and link travel time on each road together with map information related to the roads. The statistical traffic information is information generated by classifying past traffic information by types of day, such as weekday, holiday, long consecutive holidays, and averaging it by period of time. Referring to this information, an in-vehicle terminal can search for a route excluding ordinarily congested areas and further enhance the accuracy of estimation of required travel time for going to a destination.
However, situations of a traffic jam and link travel time largely vary with weather factors as well as types of day. Even when types of day are identical, therefore, patterns of a traffic jam can be expected to differ from each other. An example will be given. An area around a railroad station in the morning and evening on a weekday tends to be more congested when it is rainy than when it is fine. Conversely, an area around an outdoor facility on a holiday tends to be more congested when it is fine. Just a type-of-day factor cannot account for these tendencies, which a weather factor largely influences. To eliminate this instability of route search due to a weather factor, Japanese Patent Application Laid-Open Publication No. 2002-206936 discloses a method for search for a route with a weather factor taken into account. In this method, an in-vehicle terminal stores statistical traffic information by weather conditions, which is obtained by classifying past traffic information by type-of-day and weather factors (for example, fine weather or rainy weather) and taking statistics on the classified information. Further, the route search executed in the method refers to weather forecast information related to each area and acquired externally or weather information detected by a wiper on the vehicle and so on.
Unfortunately, the above technique, requiring a communication device and a sensor device to externally obtain the weather forecast information and the weather information, makes the configuration of an in-vehicle terminal complex. Moreover, since weather forecast information is referred to during the route search, if the weather forecast is not right and the weather is different from the expected one, the estimation of required travel time gets less accurate, reducing the accuracy of the route search.
An object of the invention is to provide a device and method for searching a route to a destination in which, without referring to weather forecast information or weather information, required travel time barely varies by the weather.